Double-Star Systems Cycle Between Big and Small Blasts

Carnegie Institution of Washington News Release

2007 March 7

Pasadena, Calif. -- Certain double, or binary, star systems erupt in full-blown
explosions and then flare up with smaller bursts, according to new information
gathered by NASA's Galaxy Evolution Explorer (GALEX) and analyzed by a team of
astronomers, including postdoctoral researcher Mark Seibert of the Carnegie
Observatories.

The data bolster a 20-year-old theory suggesting that double star systems
experience both explosion types, rather than just one or the other. It also
implies that the systems cycle between blast types, hiccupping every few weeks
with small surges and experiencing giant outbursts every 10,000 years or so.

The discovery, appearing in the March 8 issue of the journal Nature, centers
around a binary system called Z Camelopardalis (Z Cam). Astronomers have long
known Z Cam to be a cataclysmic binary—a system that features a collapsed, dead
star, or white dwarf, which sucks hydrogen-rich matter from its companion like
a stellar vampire. The stolen material forms an orbiting disk of gas and dust
around the white dwarf.

Astronomers divide cataclysmic binaries into two classes—dwarf novae, which
erupt in smaller, "hiccup-like" blasts, and classical novae, which undergo huge
explosions. Classical novae explosions are 10,000 to one million times brighter
than those of dwarf novae, and they leave behind large shells of shocked gas.

In 2003, Seibert examined ultraviolet images collected by GALEX during its
Survey of Nearby Galaxies. He noticed a never-before-seen arc and linear
features surrounding Z Cam that indicated the presence of a massive
shell—evidence that the dwarf nova had in fact undergone a classical nova
explosion a few thousand years ago. The features had remained invisible up to
this point because they cannot be easily detected at optical wavelengths.
However, they are easily seen at the ultraviolet wavelengths detected by GALEX.

"You could actually see it immediately, but we had to convince ourselves that
we were really seeing a nova remnant," Seibert said. "If true, it would
represent the largest nova remnant yet known. But it was especially shocking to
find it associated with such a diminutive dwarf nova system. Everyone was
skeptical and it took a considerable amount of time and effort to be certain."

About 530 light years from Earth, Z Cam was one of the first dwarf novae ever
detected. For decades, observers have watched the system hiccup with regular
outbursts. It brightens about 40-fold every 3 weeks or so, when an instability
causes some of the material drawn by the white dwarf to crash onto its surface.
Theory holds that Z Cam and other recurring dwarf novae should eventually
accumulate enough matter and pressure from their swirling disks of hydrogen to
trigger gigantic classical novae explosions. But no one had found definitive
evidence for this until Seibert's discovery in 2003.

Other team members confirmed that the structures detected by GALEX were indeed
parts of a massive shell of gas surrounding Z Cam. Narrowband images from Kitt
Peak National Observatory near Tucson, Ariz., Palomar Observatory near San
Diego, Calif., and the Wise Observatory near Mizpe Ramon, Israel, along with
optical spectroscopic measurements made at the Lick Observatory near San Jose,
Calif., contributed to this verification.

"The new images are the strongest evidence yet in favor of the cyclic evolution
scenario of these binary stars," said lead author Mike Shara of the American
Museum of Natural History in New York. "It's gratifying to see such strong
evidence for this theory finally emerge after all this time."

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Caltech leads the GALEX mission and is responsible for science operations and
data analysis. NASA’s Jet Propulsion Laboratory, a division of Caltech, manages
the mission and built the science instrument. GALEX was developed under NASA's
Explorer Program, managed by Goddard Space Flight Center in Greenbelt, Md.
Funding for the mission was provided by NASA.

In addition to the Carnegie Observatories, Caltech, and the American Museum of
Natural History, co-authors on the paper represent UCLA, Columbia University,
Indiana University, Wise Observatory at Tel-Aviv University, and WIYN
Observatory in Tucson, Ariz. Researchers sponsored by Yonsei University in South
Korea and the Centre National d'Etudes Spatiales (CNES) in France also
collaborated on the mission.

The Carnegie Institution of Washington (www.carnegieinstitution.org), a private
nonprofit organization, has been a pioneering force in basic scientific research
since 1902. It has six research departments: the Geophysical Laboratory and the
Department of Terrestrial Magnetism, both located in Washington, D.C.; The
Observatories, in Pasadena, California, and Chile; the Department of Plant
Biology and the Department of Global Ecology, in Stanford, California; and the
Department of Embryology, in Baltimore, Maryland.